DETROIT (AP) — U.S. safety regulators are investigating whether a recall of 156 fire trucks last year was enough to fix a problem with aerial ladders that can unexpectedly fall.

The probe covers trucks from the 2000 to 2012 model years made by Ohio-based Sutphen Corp.

The National Highway Traffic Safety Administration says two ladders fell after the trucks were recalled in May of last year. Six people were hurt in incidents in Belle Valley, Pennsylvania, and Hall County, Georgia.

Sutphen says in a July statement posted on its website that it is telling fire departments not to use the ladders until further notice. The company says it’s investigating and working to fix the problem. Messages were left Monday for a company spokesman.

The government says cables that control the ladders can fail.

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